man: use manpages.ubuntu.com for resolvconf(8) link

man: use manpages.ubuntu.com for resolvconf(8) link

Add manpages.debian.org as a man citeref "project", so we can refer to man
pages hosted there. resolvconf(8) doesn't seem to appear on any of the ones we
currently have defined.

We are doing a trick, where our man page is installed as resolvconf(1), and
we refer the reader to resolvconf(8). This can be pretty confusing, so the
least we can do is to provide a non-broken symlink ;)

v2:
- link to manpages.d.o instead of manpages.ubuntu.com, because debian is the
  upstream here, and because with debian we can use /unstable/ as the version,
  and ubuntu doesn't seem to have any such shortcut, so the version would need
  to be periodically updated.

(cherry picked from commit 42ecca2e6839aa4a36e335564461731ceee9180a)